The present invention relates to a non-contact type liquid level sensor and, more particularly, to a non-contact type liquid level sensor which can convert the motion of a float into a change in magnetic flux for detection with high accuracy and which is hence appropriate for an application to an automotive fuel tank and the like.
For example, in a non-contact type liquid level sensor, which is installed in an automotive fuel tank of an automobile for detection of the volume of stored liquid fuel, an annular magnet adapted to rotate in response to the movement of a float is disposed within a frame. A Hall-effect element, which is a magnetoelectric converting element, is disposed within the same plane as a plane where the annular magnet is disposed and at a central portion of the magnet, and the Hall-effect element detects a change in magnetic flux density due to the rotating magnet and then converts the change so detected into an electric signal, whereby a liquid level is designed to detected (for example, refer to Patent Document 1).
In addition, there is a non-contact type liquid level sensor in which a magnet holder, to which the magnet is fixed, is rotatably held by a holding means formed on a surface of a frame, and a pair of cores and a Hall-effect element provided in a gap portion between the cores are disposed within the frame so as to detect a change in magnetic flux density due to the rotation of the magnet (for example, refer to Patent Document 2).
In the non-contact type liquid level sensors disclosed in the patent documents 1 and 2, the Hall-effect element is disposed within the same plane as the plane where the magnet is disposed (Patent Document 1), or the magnet holder is rotatably held by the holding means formed on the surface of the frame (Patent Document 2), so that the thickness and production cost of the non-contact type liquid level sensors are attempted to be reduced.
In addition, in both of the non-contact type liquid level sensors disclosed in the patent documents 1 and 2, the Hall-effect element detects a change in magnetic field of the magnet adapted to rotate in response to the movement of the float and then outputs a magnetoelectric conversion signal (an electric signal) corresponding the change so detected. Due to this, the non-contact liquid level sensor and the Hall-effect element in the interior of the sensor are, as shown in FIGS. 6 and 7, is exposed directly to an external magnetic field from the periphery thereof (an external magnetic field) to thereby be subjected to effects thereof. This external magnetic field results from electric equipment and system in the vicinity of a portion where the fuel tank is placed and a disposition system, in addition to a fuel pump module P including a fuel pump which is placed within the fuel tank. Then, this external magnetic field imposes direct effects on the Hall-effect element within the non-contact type liquid level sensor 20.    Patent Document 1:JP-A-2002-206959 (pages 4 to 5, FIG. 1)    Patent Document 2:JP-A-2002-206945 (pages 3 to 4, FIG. 1)
However, an external magnetic field like this changes the output characteristics held by the Hall-effect IC itself and deteriorates the detection accuracy of a liquid level. Consequently, normal output characteristics that would be obtained in a state where no external magnetic field exists cannot be obtained. FIG. 8 illustrates output characteristics of a Hall-effect IC with and without an external magnetic field. According to what is illustrated therein, when there exists no external magnetic field in the vicinity of the portion where the fuel tank is placed, an output of a predetermined sensitivity can be obtained with respect to the rotational angle of the magnet as shown by a solid line X, whereas when there exists an external magnetic field, the external magnetic field acts on the Hall-effect IC and an output therefrom changes as shown by a dotted line Y. As a result, there are caused problems that the output change with respect to the rotational angle of the magnet is small, that the detection sensitivity is deteriorated and that the result of measurement of a liquid level lacks reliability.